In the known wireless transmission processes based on modulating an SHF carrier, the signals transmitted by the transmitter reach the receiver along a plurality of separate paths. This results in interference at the receiver which may cause fadings of the signals transmitted and consequently a loss or degradation of the information to be transmitted.
FIG. 1 illustrates an example of the swings in the power received by a receiver according to a prior art process in a frequency band extending from 5.7 GHz to 5.9 GHz. As may be seen in this figure, the power curve exhibits several sudden drops signifying breaks or degradations in the transmitter-receiver link. The frequencies corresponding to the power minima vary according to the spatial position of the receiver, and for a given position, the power level may change over time as a function of the environment which may be modified by the movement of people for example. Such breaks or degradations are unacceptable in applications such as the transmission of sound between a television set and loudspeakers for example.
The solutions used in the prior art to solve this problem are generally based on techniques such as space diversity or frequency diversity. Space diversity consists in using a transmitting antenna and a plurality of receiving antennas spaced apart in such a way as to decorrelate the signals received. In addition to multiplying the number of receiving antennas used, this technique requires the use of a complex device for combining the signals received by the various antennas.
The technique based on conventional frequency diversity consists in using a separate transmitter for each carrier frequency. Moreover, good stability of each of the frequency sources is often necessary. Finally, in order to avoid the simultaneous fading of the various carriers, the width of the frequency band separating the said carriers should be greater than or equal to the width of the coherence band of the wireless channel used. Thus, within the coherence band, the signals received are very strongly correlated, and the effectiveness of conventional frequency diversity is very strongly diminished. Now, the width of the coherence band depends on the environment in which the transmission of the SHF signals is performed. Also, the diversity of conventional frequencies used in a given environment is not always suited to a different environment.
The purpose of the invention is to alleviate the drawbacks of the prior art. According to the process of the invention, the SHF carrier is amplitude-modulated by a VHF carrier of frequency f.sub.AM, previously frequency-modulated by a signal S.sub.1 representing the information to be transmitted, and simultaneously, the variations of the frequency f.sub.p are controlled by a sinusoidal signal S.sub.c of frequency f.sub.FM.
By virtue of this process, the power received by the receiver is equivalent to the average of the powers received via the various carriers of the spectrum spread over the band B. Thus, the level of the signal received is held relatively constant, avoiding sudden fadings due to interference resulting from the multiple nature of the paths through which the signals are sent in the SHF range.
The process according to the invention is implemented by means of a transmitter including three stages mounted in cascade, namely an RF (radio frequency) stage for amplification/filtering of the signal S1, a VHF stage for frequency-modulation of the VHF carrier and an SHF stage for amplitude-modulation and frequency-modulation of the SHF carrier.